The invention relates to a method and apparatus of detecting gases and more particularly to a hydrogen sulfide detecting method and apparatus with increased longevity.
The need for reliable, inexpensive devices for detecting hydrogen sulfide and other gases has long been recognized. Sensors using semiconductors take advantage of the fact that certain substances adsorbed by a semiconductor influence the conductivity of a thin segment of film near the surface of the semiconductor. These devices generally comprise a metal oxide semiconductor mounted on an inert substrate. Dopants or impurities with a higher or lower valence are sometimes added to the metal oxide to affect conductivity. Additionally, catalysts are sometimes placed on the surface of the semiconductor to promote a reaction in the surrounding gas and so aid in detection.
A variety of these devices are disclosed in the patent literature. For example, Shaver, U.S. Pat. No. 3,479,257 discloses a hydrogen and reducing gas detector comprising a substrate made of quartz, alumina, or other substances, covered by a semiconductive thin film comprised of a metallic oxide. The metallic oxide is preferably an oxide of a metal selected from the group consisting of tungsten, molybdenum, chromium, niobium, nickel, iron, titanium, and the like. Thin film islands of a catalytic element are deposited upon the semiconductive film.
Boardman, U.S. Pat. No. 3,901,067 discloses a hydrogen sulfide detector with a semiconductor film of stannic oxide including indium as a possible dopant.
Chang, U.S. Pat. No. 4,030,340, employs a film of palladium deposited onto a semiconductor film in sensing hydrogen gas. An activation film which is preferably palladium, but also may be a palladium-gold alloy, is deposited as a myriad of discrete islands on a semiconductor film comprising stannic oxide with a dopant such as indium. The palladium or palladium-gold film acts as a catalyst to activate or dissociate diatomic hydrogen molecules.
Langley, U.S. Pat. No. 3,567,383 discloses a hydrogen detector with a thin film preferably of palladium oxide; while Fehlner, U.S. Pat. No. 3,793,605, discloses an ion sensitive device which employs a classical ferroelectric material to support a spray of gold islands.
These and other detectors suffer from one or more of several limitations. For example, many prior devices fail to provide sufficient longevity, while other devices are set to primarily detect hydrogen or gases other than hydrogen sulfide. Other devices rely on catalysts to break up the gases to be sensed. However, the catalysts are subject to poisoning, such that the sensor has a limited life. Still other devices are substantially detrimentally affected by humidity, while others require the pretreatment of the gas to remove hydrogen sulfide which slows response time. Still other devices are said to require repeated exposure to hydrogen sulfide if the device's efficacy as a sensor is to be maintained.
These and other limitations of prior processes and methods are substantially minimized, if not eliminated, by the present invention.